I haven't used RS for my daughter who is learning Spanish but she goes to a Spanish Immersion program. I have to disagree with your speech therapist because learning a second language tends to concrete your understanding of grammar in your native language. I would be curious to read what her/his argument is based on. I know learning a second language can slow down a child's own language but this is a short term issue and one that experts argue is a viable concern when the child is younger than 2 yrs of age. But this does not consider children with 2E concerns.
Have you looked at other options besides RS? We have Muzzy and though I don't think someone will become fluent with the program alone; it is something my child enjoys.
